Today, September 14, 2011, is the vernal equinox for the northern hemisphere of Mars!

If you want to be technical, it’s the time when the axis of Martian rotation is perpendicular to the direction of the Sun, and the northern hemisphere is headed into summer (making it the autumnal equinox for the southern hemisphere). When this happens here on Earth, it’s called the first day of spring (here in the US at least, in other countries it’s considered the middle of the season — a tradition with which I agree).

Mars, like Earth, is tilted with respect to its orbit around the Sun; Earth is canted at an angle of roughly 23°, while Mars is at 25°. That’s why we (and Mars) have seasons! Over the course of the year, the angle of sunlight hitting the surface of the planet changes, heating it more efficiently in the summer and less in the winter. Boom! Seasons.

On Earth, that’s most dramatically seen as polar ice shrinks and grows over the year. Same with Mars! The picture above is from the European Space Agency’s Mars Express orbiter, and shows the northern polar cap in May 2010, during the last summer. The north pole of Mars is icy, but it’s actually two kinds of ice: water, and frozen carbon dioxide. CO2 turns directly from a solid into a gas (a process called sublimation), and does so at much lower temperature than water ice melts. This means the CO2 goes away first as temperatures rise in the northern hemisphere, leaving the water ice behind. In that picture, the ice is essentially all water.

Where does the CO2 go? Into the atmosphere! So much of it adds to the air there that the atmospheric pressure on the planet increases measurably, by about 30%. That much extra carbon dioxide adds a small but significant greenhouse effect to the planet as well, warming the surface. Estimates vary, but I’ve seen quotes of a few degrees Celsius for the effect.

I’ll note that the year on Mars is about twice the length of an Earth year, so all the seasons are about twice as long as well. May 2010 is when summer started on Mars (in the northern hemisphere), and here we are 17 months later with the start of spring. The Planetary Society has a page listing the next few seasonal dates on Mars if you’re curious.

So anyway, happy first day of spring, Martians! Don’t forget to try to stand up malagor eggs today.

If we aver finally settled on Mars, assuming religion followed us there, I wonder how long it would take until terrestrial-based religious observances would be abandoned in favor of Mars-based dates…
That is, I’m making an assumption that humans would revert to ‘annual’ observances based on the local year, being the pagan creatures we are.
Maybe then, with Christmas, Easter, Ramadan, etc decoupled from Earth’s, Martian society would become distinct from Earth.

@kevbo
I can hear the conversation now
Child: “Mom, it’s Christmas on Earth, why don’t I get any presents?”
Mom: “Honey, Santa can only come to Mars every 2 Earth years because we are so far away.”
Child: “I hate Mars”

The Martian seasons are not only opposite of each other in the two hemispheres, they’re of vastly different severity.

Mars does one thing the Earth does not: It gets substantially CLOSER to the sun in certain parts of its orbit than in others. It’s orbit is very eccentric. While the the Earth’s aphelion distance is only about 3% farther out than Earth’s perihelion distance, Mars’s aphelion distance is a whopping 20% farther out than Mars’s perhelion distance.

As on Earth, Martian perihelion corresponds very closely with the Northern Hemisphere’s Winter Solstice. This means that the Winters and Summers in the Northern hemisphere of Mars are pretty mild, but the Winters and Summers in the Southern hemisphere of Mars are made vastly, hugely, mind-bogglingly more severe.

Southern seasons on Mars would be like living in the Canadian outback during the dead of winter, then living in the Sahara at the height of summer.

Given the lower gravity and different surface area of Mars, how much gas would have to be added to the planet to give it an approximation of earth atmospheric pressure at human-livable temperatures? And if that added gas were all frozen into a ball (say some moon or outer solar system object) what would its radius be? We could ‘just’ (devil in the details) dump one of these onto Mars- the heat from impact would nicely warm the planet. Beats the heck out of waiting forever using slower processes.

So enough frozen co2 sublimates to raise the atmospheric pressure by a whopping 30% and whiles estimates vary you have seen quotes OF UP TO a few degrees celsius for the temperature rise (due to the greenhouse effect). If those figures are correct seems to point to co2 as a very weak GHG. I would have expected a higher temp rise from the increase in atmospheric pressure alone

While we are at it, I highly recommend Kim Stanley Robinson’s Mars Trilogy books. It depicts the colonization and terraforming of Mars, while trying to stay as scientifically correct and realistic as possible.

@ ^ Mephane : Seconded by me. Great books and interesting – mostly really good and plausible science in them too.

@ 2. csrster :

Happy First Day of Autumn and Fair Dinkum’ me ol’ Billabong to any austalo-Martians reading.

Indeed. Don’t forget one hemisphere’s spring is another hemisphere’s autumn. Well spotted csrster.

Oh & G’day from Down Under / Up Over depending on your Point-o-view! 😉

@ 12. tracer :

.. Mars does one thing the Earth does not: It gets substantially CLOSER to the sun in certain parts of its orbit than in others. It’s orbit is very eccentric. While the the Earth’s aphelion distance is only about 3% farther out than Earth’s perihelion distance, Mars’s aphelion distance is a whopping 20% farther out than Mars’s perhelion distance. .. [Snip] .. Southern seasons on Mars would be like living in the Canadian outback during the dead of winter, then living in the Sahara at the height of summer.

Except far more extreme temps~wise (at the cold temp end anyhow) & without the breathable atmosphere or faintest hint of biosphere. 😮

“When this happens here on Earth, it’s called the first day of spring (here in the US at least, in other countries it’s considered the middle of the season — a tradition with which I agree). “

Certainly not in all other countries. Off the top of my head, I can’t think of anything. Yes, Midsummer in Sweden is celebrated on the weekend near the solstice, but in other contexts the seasons there start at the same time as in the US. What countries are you thinking of?